By-passes to save limbs may be required to have a relatively long length, such as the distance from the groin to below the knee, and may be required to attach to arteries that may be as small as 1 mm to 5 mm in diameter. Where patients have no other veins that can be used, as is often the case with patients having relevant serious conditions, the only positive alternative is to use prosthetic grafts of synthetic materials, for example flexible tubes of polytetrafluoroethylene (PTFE). Simple direct end connections or anastomosis of prosthetic graft tubes (usually run at an acute angle or more or less parallel with the artery and end cut at an angle) to side apertures in arteries, perhaps particularly arteries substantially less than 5 mm in diameter, has unfortunately been followed by formation of fibrous intimal hyperplasia, which leads to serious blood flow reduction and even stoppage. The fibrous intimal hyperplasia occurs in regions within and around the graft connection, where there is little or no shear stress between the blood flow and the graft and arterial walls.
It is known to use a small piece of natural vein to make a short cuff known as the Miller cuff, that is joined by surgical stitching to and between the artery opening and the end of the prosthetic graft tube. Improved success rates for indirect prosthesis-to-vein-to-artery connection, compared with direct prosthesis-to-artery, have involved reduced adverse effect from intimal hyperplasia. Contributory factors, for cuff type and other prosthesis types, have been considered and postulated as including reducing tendencies to turbulence of blood flow, and/or optimizing approximation to laminar blood flow, and/or for suppleness of the natural vein parts to aid absorption or cushioning blood pulsing. These factors have further been seen particularly as contributing to avoiding or minimizing occurrence of artery wall shear stress. However, fibrous intimal hyperplasia still occurs with the so called Miller cuff because regions of flow separation and low shear stress still occur within the cuff.
U.S. Pat. No. 5,156,619 discloses a vascular prosthesis comprising a tube of material other than autologous vascular tissue, the tube having an enlarged end formation for surgical connection direct to an opening formed in an artery, the formation having a heel and a toe at opposite ends of a first longer diameter parallel to the axis of the tube and a second shorter transverse diameter. WO 97/31591 discloses a flanged graft for end-to-side anastomosis grafting having an integral terminal flanged skirt or cuff, which facilitates an end-to-side anastomosis directly between an artery and the expanded flange bypass graft without need for an intervening venous collar or venous patch.
It has been proposed to provide a vascular prosthesis comprising a tube of synthetic material having an end formation for surgical connection directly to an opening formed in an artery, the end formation comprising an enlarged chamber serving to promote localized movement of blood having a non-laminar nature with a shear stress inducing relationship to the arterial wall. The term “non-laminar” as used herein is intended to define blood flow other than parallel to arterial walls and, in particular, includes localized laminar movement of blood having significant secondary components. The proposed enlarged chamber has a convex outer wall. Further experimentation, however, has revealed that this type of vascular prosthesis, while representing an improvement on the aforementioned Miller cuff may still not be ideal for certain applications.
Therefore, it is an object of the present invention is to provide an improved vascular prosthesis for use in vascular surgery.